[Qt-interest] Performance issues with Graphics View Framework

[Enrico Ros]

On Tuesday 21 July 2009 10:20:54 Simone Artesino wrote:
> Ok Enrico, but let's consider now these tests result, made on atom chipset
> polusbo. running a 30 fps timer,
> when use as viewport a Widget I obtain 30 fps and 19% cpu load.
> when use as viewport a QGLWidget I obtain 5 fps and 67% cpu load.
> these results are not influenced from switching viewport update policy.
>
> What I can infer from this result?
> my qpixmap copy is well hardware accelerated when I use a Widget (and is
> stored in gpu internal memory I guess), instead when I use a QGLWidget the
> copy is translated in a very bad open-gl code that makes several copies of
> memory from ram-cpu memory to gpu.
>
> Is a correct deduction? what you think about it?

It seems so, yes. Moreover, there may be pixel conversion stuff involved (RGB 
(qimage/qpixmap) to BGR (opengl), I think). Uploading a texture to OpenGL 
takes time, maybe it's less accelerated than a simple memory->gpu pixmap 
transfer.

In any case if you want to get the maximum performance for you application, 
use the old school trick I explained you in the previous mail. Just upload the 
pixmap once to the OpenGL memory and draw a fixed rectangle with scrolling 
texture coordinates on vertices (to select the right 'scroll portion').

Without looking at the code this is the best I can tell.

Ciao,
Enrico

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